1. Field of the Invention
The present invention is directed generally to a closed drift Hall type accelerator in a vacuum space.
2. Description of Related Art
The present invention attempts to achieve a high energy ion beam source which is superior to existing accelerators in beam density and energy. Hall Current accelerators operate without space charge limitations as are present in conventional ion beam accelerators, which use electrostatic lens (grids) to extract ions from a neutral plasma (gridded ion sources). Such devices can have very good optics. That is they can have a very well defined velocity vector and energy spread.
In Hall accelerators ion acceleration is achieved by providing a voltage potential between an anode associated with a neutral gas source and a cathode in the general vicinity of the beam exit. Electrons from the cathode migrate through the Hall effect fields, which pick up the electrons in E×B azimuthal orbits, restraining their axial transit. This allows electrons to accumulate in the Hall magnetic fields. The neutral gas from the anode structure encounters the counter-streaming electrons and becomes electron impact ionized, thereby forming ions that are accelerated by the electric field between the anode and the cathode. The ions are never separated from the electrons as in electrostatic extraction. An accelerating electric gradient is established between a virtual cathode formed by the electrons captured in Hall effect orbits and the anode. Closed drift Hall effect accelerators are generally defined as having ion gyro-radii much greater than the acceleration channel width and electron gyro-radii generally somewhat less than the channel width. Thus the ions go through minor azimuthal deviation in the acceleration Hall effect field. Most Hall effect devices have been of the single stage type, where the neutral gas is introduced into the ionization and acceleration region from within the Hall effect magnetic field structure. Therefore the ions are accelerated only through the exit fringe field of the solenoid. Transport through the fringe (radial component of the solenoid field) generally imparts azimuthal momentum to the ions. Two stage accelerators have been studied in an attempt to reduce the divergence of the beam because the azimuthal thrust imparted to the ions in the first fringe field is countered by the second fringe field which is in the opposite direction and therefore imparts opposite azimuthal thrust to the ions. Two stage accelerators contain inner and outer pole pieces that generate the two alternately directed radial fringe fields.
To inject ions into the end of a solenoid, they must be transported across the fringe magnetic field. Axial ion injection has been most frequently attempted by means of magnetically insulated pulsed type ion diodes. The ions generated by such devices are then typically transported across a full cusp, not a half cusp (fringe) field. A full cusp exists between two oppositely aligned solenoids. Ion transport across a full cusp results in large canonical angular momentum orbits that do not approach the magnetic field axis but rather encircle the axis. The prior art ion injection methods are limited by issues of ion trapping as well as space charge neutralization. Generally trapping schemes involve fast magnetic field ramping or a change in the charge to mass ratio by molecular disassociation. Prior art methods for space charge neutralization typically involve the pre-introduction of background plasma because electrons are stripped from the beam as the ions transport across the field.
The present invention is distinguished from previous methods of cross field transport into a solenoid field in that the present invention actively accelerates ions across the fringe field, utilizing the fringe field itself as a Hall effect ion acceleration field. Because the accelerator operates on the Hall effect principle electrons are present everywhere, mitigating space charge issues, both in the fringe and in the solenoid. Additionally, since the ions are accelerated into the field they will be decelerated if they attempt to escape, and ideally returned by the initial process. Hall acceleration occurs across the fringe field. When the electron source cathode is located outside of the solenoid, beyond the distal (exit) fringe, then both fringe fields serve as Hall effect fields and the ions are accelerated out of the distal end of the solenoid. The azimuthal momentum imparted by the first (entrance) fringe field is countered by the second (exit) fringe field, returning the original axial trajectory to the ions. The ions will have gained energy by acceleration across both fields.
The present invention reveals one or more electron sources positioned at any desirable location around or within the solenoid magnetic field. The electron sources neutralize ion space charge, from the time of ion formation from neutral gas throughout the acceleration and storage process.
The present invention is further distinguished from the prior art in that the ion gyro-radii may be generally equal to the axial length of the acceleration channel. The ions are made to undergo a generally 90 degree angle as they pass through the Hall effect field. The initial axial trajectory is bent into azimuthal during the entrance acceleration process. Within the solenoid the ion orbit should have very low canonical angular momentum. An ion with low canonical angular momentum, passes close to or crosses the magnetic field axis. This much greater ion bending is made possible because the inner pole piece has been removed and the ions are free to gyro-rotate too the axis of the magnetic field once they have entered the solenoid.
In a present embodiment the ions should have low, desirably zero, values of canonical angular momentum. An ion that transports across one end fringe of a solenoid can have zero canonical angular momentum if the ion was originally traveling parallel to the magnetic field axis. Zero canonical angular momentum orbits are characterized by having contact with the magnetic field axis once each orbit. The apparatus is preferably designed such that the ions exit the solenoid at a point in the solenoid where the ions are on axis. Zero canonical angular momentum ions will exit the solenoid with maximum axial momentum, and minimum radial momentum. Thus a well collimated beam emerges and the initial annular distribution of ions is combined into a small cross section central axis ion beam. Hall Current accelerators are of annular design because of the need for radial magnetic field lines to restrain the electrons in closed drift Hall Current orbits capable of producing the accelerating electric field gradient. The Hall effect electrons also provide ionization to the neutral gas.
The present invention is further distinguished from the prior art in that an annular gas valve is implemented such that the gas entering the radial Hall effect fringe magnetic field is a cylindrical sheath of collimated neutral gas, thereby minimizing any non-axial momentum particles. Neutral gas particles encounter the radial Hall effect field traveling parallel to the axis but not on axis, as is required for a Hall accelerator. The annular well collimated axial neutral gas sheath is formed into a stream of spiraling ions in the solenoid.
The present invention is further distinguished from the prior art by its ability to combine the annular geometry Hall effect beam into a single central beam in the exit fringe field. This is possible because there is no inner pole piece. The beam collimation is determined by the collimation of injected neutral gas.
The following references illustrate the prior art with regard to Hall Current accelerators. Raitses et.al. U.S. Pat. No. 6,448,721 reveals an example of the progression of Hall accelerators from the common annular design towards a reduction of the inner electrode with their cylindrical geometry Hall accelerator, the design references a single stage Hall accelerator. Fisch et.al U.S. Pat. No. 6,777,862 discloses a segmented electrode Hall thruster with reduced plume, addressing the importance of reducing plume divergence. Mahoney et.al. U.S. Pat. Nos. 5,973,447 and 6,086,962 discloses a gridless Hall effect ion source for the vacuum processing of materials. I. P. Zubkov et.al. in OPTIMIZATION OF A HIGH CURRENT ION ACCELERATOR, Soviet Physics—Technical Physics, Vol. 17, No. 2 October 1972, reveals how a two stage Hall accelerator has an appreciably reduced angular beam divergence. Kornfeld et.al. in U.S. Pat. Nos. 6,523,338 and 7,075,095 discloses plasma accelerators using multi-acceleration stages. Cann U.S. Pat. Nos. 3,309,873 and 3,243,954 discloses a plasma accelerator utilizing a Laval type nozzle and Cann U.S. Pat. No. 3,388,291 discloses an annular array of multiple collimating Anode gas sources, but not an annular structure. Kapetanakos U.S. Pat. No. 4,293,794 reveals a method of pulsed full cusp cross field transport of ions into a solenoid field, with a half cusp beam exit. Maglich U.S. Pat. No. 4,788,024 reveals a high energy, low current injection that attempts to achieve zero canonical angular momentum orbits. J. R. Pierce U.S. Pat. No. 2,847,607 discloses a beam focusing apparatus. Erwin Becker U.S. Pat. No. 3,628,342 reveals a method for fluid gas separation utilizing an annular nozzle. Kaufman et. al. US2002/0163289 A1 discloses a single stage Hall effect accelerator. C. E. Berry U.S. Pat. No. 2,672,560 reveals an annular ionization chamber. Jassby et. al. reveals a particle beam injection system. Wells U.S. Pat. No. 4,267,488 discloses a system for forming and compressing plasma in axis encircling ringlike toroidal plasma vortex structures. H. C. cole in A HIGH CURRENT HALL ACCELERATOR, Nuclear Fusion, 10, 1970 reveals a high current two stage Hall accelerator. I. P. Zubkov in EXPERIMENTAL STUDY OF A TWO-LENS ACCELERATOR, Soviet Physics—Technical Physics, Vol. 15, No. 11, May 1971 discloses a high current closed drift Hall accelerator.